45/70 barrel length velocity loss ?

[deathwobble]

Ok recently got a HR 45/70 ultra 24"stainless rifle. Although I love the feel of the rifle very basic and simple it kicks like a frieght train. So i have decided to have a muzzle break put on this rifle... I was wondering how much of a velocity loss I can expect If I have my gunsmith shorten my barrel to 18" over all with the break so looking at about 7" of barrel lopped off. Im not really concerned about affecting the trajectory of the rifle as much as the 100-200 yard stopping power.

currently loading
powder: H335
Charge: 68gr
Bullet: 300gr JHP
Primer: fed215 LRM
Velocity: 2505fps

thanks